1. Field of the Invention
The present invention is related to materials processing machine components, particularly cylinders used therefor. Still more particularly, the present invention relates to a materials processing cylinder containing titanium carbide.
2. Description of the Prior Art
Materials processing cylinders, such as used for injection and extrusion machines, plastics injection molding and extrusion processes, chemical processes, rubber, elastomer and magnesium processing, and food processing, are subjected to high temperatures, wear and corrosion attack from the materials being processed. Further, screws used for processing work material (as for example plastic) cause galling as the screw rotates within the cylinder. Additionally, such cylinders must also exhibit high strength, on the order of withstanding 30,000 psi at 1,000 degrees F. Still further, such cylinders must be cost effective and yet be highly resistant to failure when exposed to wear, corrosion, high pressure and high temperature.
Since there is no material known which meets all of the above requirements, the art has adopted the following manufacturing techniques.
A first technique is known as a "nitrided cylinder". As shown at FIGS. 1 and 1A, a solid, single piece cylinder 10 is provided, composed of nitrided steel. The inside surface 12 of the cylinder 10 is nitrided and then machine polished to provide a nitrided skin 14, typically about 0.010 inches thick. Problematically, the nitrided skin 14 is easily spalled off, has poor corrosion resistance, and has high manufacturing cost for larger size cylinders.
A second technique is known as a "bimetallic cylinder", and is the most popular technique. As shown at FIG. 2, a composite cylinder 20 is provided having an outside layer 22 composed of a high strength steel, and an inside layer 24 composed of an inlay alloy metallurgically bonded thereto and typically about 0.065 inches thick, and wherein the outside layer 22 serves as a backing for the inside layer 24.
The centrifugal casting process used for forming the inside layer 22 is shown at FIGS. 3A and 3B. An inlay alloy material in the form of shots, ingots or powder 26 is distributed along the inside surface 28 of the outside layer 22. Then, the outside layer 22 is slowly rotated and heat Q is applied to achieve a temperature above 2,200 degrees F. The outside layer 22 is then spun at room temperature to provide, for example, a centrifugal force of over 70G's. During this process, the inlay alloy melts and forms a uniform coating, which upon cooling, provides the inner layer 24. Typical inlay alloys are: Fe--Ni--B, having a hardness of Rc 60-65, and having good wear resistance, but poor corrosion resistance; Co--Ni--B, having a hardness of Rc 48-55, and having good corrosion resistance, but poor resistance to abrasion; and, carbides, mostly with tungsten carbide powder bound by a Ni--Cr--B matrix alloy with a lower melting point, which inlay alloy is superior to the first two inlay alloys, as described in U.S. Pat. No. 3,836,341 and 4,430,389 which are hereby herein incorporated by reference.
Due to an increase in the use of technical plastics containing abrasive and corrosive additives, such as high glass fiber fillings, pigments, UV stabilizers, and flame retardants, the inside surface of processing cylinders are subjected to severe wear and corrosion conditions. The bimetallic cylinders discussed in U.S. Pat. Nos. 3,836,341 and 4,430,389 utilize tungsten carbide to provide corrosion and wear resistance, but nevertheless suffer from the following disadvantages.
1. The centrifugal force of the bimetallic cylinder casting process tends to spin high density tungsten carbide to the outer wall of the inner cylinder. Consequently, there are fewer tungsten carbide particles at the machine finished inside surface of the inner cylinder.
2. Tungsten carbide has a relatively high coefficient of friction. Therefore, frictional wear of the processing screw flight surface occurs where contact is made with the inner surface.
3. Tungsten carbide has a relatively high density and high cost per pound weight.
4. Tungsten carbide does not exhibit good corrosion resistance to hydrochloric and hydrofluoric acid atmosphere attacks, a condition that is presented by most resins.
Accordingly, what is needed is a material for a materials processing cylinder composed of a material which is not subject to the above recounted disadvantages.